Using the bubble tool
Type on the left and circled letters appear on the right. Bubbles switches between the outline set and the filled set, where the letters are knocked out of a solid circle.
The filled set has capitals only, so lowercase comes back as capitals when you pick it. Punctuation has no circled form and stays as it is, listed under the output.
Two sets, 30 years apart
The outline circles live in Enclosed Alphanumerics, from U+2460. They were added early, for round-trip compatibility with Japanese character sets that already had circled numbers for numbered lists, and the letters came along with them.
The filled capitals are much newer. They sit in Enclosed Alphanumeric Supplement at U+1F150, added in Unicode 6.0 in 2010, in the same release that brought in the first large batch of emoji. That's why they behave like emoji in some apps and render in colour on a few platforms.
The filled digits come from somewhere else again. ❶ through ❾ are Dingbats, drawn by Hermann Zapf for the Zapf Dingbats typeface in 1978, and Unicode took the set wholesale. Three sources, one row of output.
The circled alphabet
What it costs you
Every filled capital is a code point above U+FFFF, so it takes two UTF-16 units. A 20-character message in filled bubbles can count as 40 in a field that measures the way JavaScript's string length does, and some apps will cut it in half.
Search engines don't fold ⓑⓤⓑⓑⓛⓔ back to "bubble", so a name written this way is close to unfindable. Screen readers announce it letter by letter, usually with "circled" before each one. It's a decoration for a display name, and a bad idea for anything you want read or found.
Related conversions
Superscript is the same idea in a different direction, swapping each letter for a dedicated small character. Upside-down text borrows letters from other alphabets to fake a rotation.
The Unicode converter shows which block each circled character came from.
Questions
What is bubble text?
Text written with Unicode characters that have a circle built into them, like ⓐ and Ⓐ. They're ordinary characters, so they paste anywhere.
Why is my lowercase turning into capitals?
The filled set has no lowercase letters in Unicode. Only the outline set covers both cases, so filled lowercase is folded up to capitals.
Does bubble text work in Instagram bios?
Yes. These are characters rather than fonts, so anywhere that accepts text will show them, subject to the platform's own character limit.
Why do numbers look different from letters?
The circled digits and the circled letters came into Unicode from different sources, and the filled digits are Dingbats from 1978. The designs were never drawn as one family.